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F. GRAY MODULATING SYSTEM March 29, 1932.

Original Filed on. e. 1925 (zoo lnvenfar:

Fran/r Gray 460 600 new Currenr (M'Mmmperes) was 29, 1932 UNITED STATES PATENT FRANK GRAY, or NEw YORK, N. Y., ASSIGNOR T0 BEL ELEPHONE mom'roams. mconromrnn, or NEw YORK, N. Y., A conronarroN or NEW Yonx MODULATING sYsTnn'r Original No. 1,858,843, dated February 14, 1928, Serial No. 60,713, filed, October 6, 1925.

' reissue filed January 9, 1930. Serial No. 419,690.

This invention relates to modulating sys-- tems and more particularly to the method and means for modulating a carrier current by means'of light variations.

The invention has for its object to provide a more eflicient system of modulating. a carrielr signaling current by means of light signa 5.

A feature of the invention relates to the method of producing a photoelectric current of an alternating character.

Another feature of the invention relates to the novel combination of a photoelectric cell with circuits and apparatus whereby, due to the varying characteristics of the cell, light signals directly modulate a carrier signaling current.

Other features and advantages which are inherent in the invention while not specifically enumerated will be revealed upon a consideration of the description given hereinafter.

Fig. 1 of the drawings shows a photoelectric cell of special construction.

Fig. 2 shows the cell of Fig. 1 employed as a modulator ofan alternating current.

Fig. 3 illustrates curves showing the relation between the external field current and the current through the photoelectric cell.

Fig. 4 is a cross section of the cell shown in Fig. 1.

When the light sensitive element of a photoelectric cell is subjected to light' rays, it is knownthat negative electrons are emitted with a velocity which is a function of the potential applied to the anode, or in other words this'velocity is a function of the .strength of field between the anode and cathode. If these discharged electrons and the ions produced by collision are then subjected to the action of an external magnetic field at right angles to their direction of motion, the number of such electrons or ions reaching the electrodes is therefore materially reduced. Thus, when a photoelectric cell' 1s subjected to'a magnetizing field in the proper direction, the value of the photoelectrlc current flowing through the cell is ,de-

'termined not only by the intensity of illumination of the light sensitive cathode but also by the strength of the field. magnetic field is alternating in character and of a fixed amplitude, the output of the photoelectric cell'is also a corresponding alterexternal' magnetic natingcurrent of the same frequency but of' OFFICE l-Vhen the current for providing the' Application for with an inert gas, which is cylindrical in I shape. Suitably supported within the vessel are the anode and cathode 2 and 3, .respec-- tively. Thccathode 3, as shown in Fig. 4,

is cylindrical and may be of any suitable material, preferably metal. The cathode is coated with'an' appro riate substance which under the action of ight emits photoelectrons. The composition of this substance and the manner of coating the cathode surface do not form features of the present invention. Accordingly, any of the hydrides of the alkali metals may be employed as the light sensitive coating, which may be applied in any wellv known manner; or the cathode ma consist of a photo-sensitive metal coate directly onto the glass walls of the vessel. The anode 2 is a metallic rod and is mounted concentrically with regard to the cathode 3. Suitable conductors 4 and Sam sealed-in through the walls of the glass vessel, to provide electrical connections to the electrodes.

Referring to Fig. 2, a description will now be given of one method'of incorporating a cell, such as shown in Fig. 1, in a signaling circuit to modulate the signaling current.

For this purpose, the cell 6 is surrounded by a magnetizing coil 7 which is connected in series with the secondary winding 8 of a transformer 9, and in series with the biasing battery 11 and the variable resistance 12. The purpose of the battery 11 and the resistance 12 will be described hereinafter. The primary winding of transformer 9 is connected to a. source of alternating signaling current 13, represented schematically. The cathode v 14 and the anode 15 are connected in series with the primary windin 16 of an output transformer 17, and in series with batter 18 and variable resistance 19. The secon ary winding of transformer 17 ma then be connected through a suitable am li ing device to the signaling line L. Sho d t e l ne L be the usual telephone line with its associated equipment, the source 13 may be such asto generate a carrier current of. the order of voice frequency. In this case, the field winding 7 may be a high impedance, low current winding, and may beprovided with an 1ron core.

The stem disclosed in Fig. 2 is adjusted in the ollowing manner:

A (predetermined amount of light, correspon -ng to the'maximum exposure of cell 6 under signaling conditions, is allowed to impinge upon the cathode 14 whereby a photoelectric current of say 50 10' amperes is produced, as indicated in .Fi 3. With switch 23 open resistance 12 is justed until the meter 24 registers 140 milliamperes as indicated by the abscissa AB of Fig. 3, thus givingba photoelectric current DB as registered y meter 26. Switch 23 is then closed and alternating current is applied to the field winding. It is to be noted that the current from source 13 flowing through the secondary winding of transformer 9 1s represented by thecurve 1 of Fig. 3, and the direct current component flowing through this same winding is represented by the curve CD of Fig. 3. The resultant current flowing through the secondary winding of transformer 9 is therefore represents by the dotted curve 2. It will be noted that this current varies between the limits 80 and 200 milliamperes respectively. It will also be observed that this current is ofsubstantially the same wave shape as that delivered by the source 13.

The circuits are now in readiness for the reception of light si als which may be controlled by the varymg transparency of an exposed film or in any other manner suitable for signaling by light rays of varying intensity.

Curve 3 of Fi 3. shows the manner in which the photoefiactric current through the cell varies with different values of current through the field 7 and with a given intensity of light incident u on the cathode 14. It will be noted that he portion EF of curve 3 may for all practical purposes be considered as linear in character. Consequently, should a field current represented by curve 2 be applied to the-winding 7, the resultant photoelectric current will vary according to the portion EF of curve 3. The amplitude of this current may thereupon be changed by the intensity of light rays incident upon cathode 14.

05 When the cell is to be employed for modulating a carrier current for picture trans- ,source 31 passes through the o ning mbaflle plate '32 and thence throug the picture film and the transparent drum. This light is then reflected by a suitable reflecting system re resented by mirror 33, and appropriatel ocused upon the cathode 4 of cell 6. hen the picture is to be transmitted, drum 29 is started in rotation and the cathode 14 is variably exposed tolight from the source 31 in accordance with the degrees of transparency of film 28. This varying quantity of li ht therefore corres ondingly modulates t e amplitude of the a ternating current flowing through the transformer 17. v

While in the foregoing'description the invention has been shown as applied to a picture transmission system, it is understood that it may be applied to any system wherein light signals are to be translated into elec- 'trlcal signals. Accordingly, the invention is to be limited only light sensitive device, means for subjecting the light sensitive device to light rays to produce photoelectrons and gaseous ions,-l10 and means for producing an electroma netic field exterior to said light sensitive evice by means of the alternating current to be modulated.

3. In combination, a photo-electric cell, a 11 source of light, means for producing in said cell under control of light from said source a photo-electric current with a rectilinear characteristic, a source of alternating current, and means energized by said alternat- I20 ing current for changing said linear photo: electric current to a photo-electric current having a wave shape substantially similar to that of said alternating current.

4. In combination, a photo-electric cell having an anode and a cathode and being filled with an inert gas, a source of light for causing the emission of electrons from said cathode, said electrons producing gaseous ions by collision and having a velocity determined by the potential of said anode, and

means external to said cell for producmg an alternating magnetic field to vary the direction and velocity of said electrons and said 5. In combination, a photo-electric sell, a source of alternating signaling current, and means for producing a photo-electric current through said cell of the same wave shape and m frequency as said alternating current, said means comprising a magnetizing c011 energized by said alternating current.

6. In combination, a photo-electric cell, a source of alternating signaling current, and

, means for producing an alternating'photoelectric current through said cell, said means comprising a high impedance field coil external to said cell and adapted to be energized by said alternating current.

20 '7. A light-sensitive device comprising a tubular vessel, an anode within and concentrio with said vessel, a cathode within said vessel comprising light-sensitive material in part at least surrounding said anode, a coil surrounding said vessel and means for energizing said coil for controlling the photoelectric current in said light sensitive device in accordance with said energization.

8. A light-sensitive device com rising a cylindrically shaped vessel, a cylindrically shaped anode within said vessel and coaxial therewith, a cathode in the form of a lightsensitive layer lying along the outer wall surface of said vessel and means comprising a a coil surrounding said vessel and coaxial therewith for setting up a magnetic field within said vessel and in a direction parallel to the axis of said vessel.

In witness whereof, I hereunto subscribe 40 my name this 30th day of December, 1929.

FRANK GRAY. 

